Compact high voltage transformer having more uniform equipotential line spacing



c. w. PARK TAGE T 3,265,998 AVING MORE AGING RANSFORMER H TIAL LINE SP 3 Sheets-Sheet l INVENTOR. CHARLES W. PARK ATTORNEY.

EQUIPOTEN COMPACT HIGH VOL UNIFORM Flled April 14, 1964 Aug. y, i966 Aug. 9, 1966 3 265,998

C. W. PA COMPACT HIGH VOLTAGE TRANSFORMER HAVING MORE UNIFORM EQUIPOTENTIAL LINE SPACING Filed April 14, 1964 3 Sheets-Sheet 2 INVENTOR. CHARLES W. PARK ATTORNEY Aug. 9, 1966 c. w. PARK 3,265,998

j COMPACT HIGH VOLTAGE TRANSFORMER HAVING MORE UNIFORM EQUIPOTENTIAL LINE SPACING Filed April 14, 1964 5 Sheets-$heet 3 INVENTOR.

' CHARLES W. PARK ATTORNEY United States Patent O 3,265,998 ACT HKGH VOLTAGE TRANSFGRMER HAV- CeR/i MORE UNHEORM EQUIPOTENTIAL LENE SPACING Charles W. Park, alrland, Calif., assignor to the United 'States of America as represented by the United States Atomic Energy Commission Filed Apr. 14, 1964, Ser. No. 359,804 9 Claims. (Cl. 336-70) The present invention relates to high voltage power :transformers and more particularly to a transformer of the 'type having solid, dry insulation which may be substantially smaller than conventional transformers of 'similar voltage and power ratings. The invention described herein was made in the course of, or under, Contract W-7405-eng-48 with the United States Atomic Energy Commission.

It is highly advantageous to reduce the overall size of a high voltage power transformer, since the additional ad vantages of lower weight and lower cost for materials are obtained along with the convenience of the small size. The principal obstacle to size reduction in high voltage apparatus is the arcing which occurs when excessively high vol-tage gradients are produced. The dielectric stresss across a high voltage coil is generally severe at the sides of the coil at the transition where the equipotential lines emerge into `air from the insulation surrounding the coil, and at such transition, dielectric stress is most likely to be concentrated at regions where the equipotential lines are closely spaced. Thus the best transformer, from the standpoint of avoiding `stress effects, is one where the equipotential Iline spacing is balanced so that the distance between lines is equalized at the transition from insulation to air. In a conventionally designed transformer,

'any regionsfhavin'g closely spaced equipotential lines. prevent significant size reduction sincethe maximum voltage gradient is quickly reached in such regions. However, in the present invention, the size of a transformer rated at the saine overall voltages as a conventional transformer may be much less than the size of a conventional transformer because the maximum voltage gradient is not limiting at any one region.

VIn the present invention, a combination of means are utilized to accomplish such equalization `of equipotential line spacing, including Winding the transformer coils to have a semi-trapezoidal cross-section and utilizing potting materials having differing values of dielectric constant in different regions of the insulation to impart a preferred configuration to the equipotential lines of the electric field. Conductive shields are embedded in the potting compound to further aid in shaping the equipotential lines.

It is an object of the present invention to provide ari improved, more compa-ct high voltage transformer of the type having dry, solid insulation.

It is another object of the present invention to provide a high voltage transformer having a more yuniform spacing between electrical equipotential lines.

It is an object of this invention to provide a very compact dry high voltage transformer with relatively high voltage and power ratings.

1t is another object of the present invention to provide a transformer having a high voltage winding which is uniquely shaped to op-timize the electric field equipotential line configuration.

It is another object of the present invention to provide a transformer in which the dielectric constant of the potting insulation is caused to differ in different regions of the transformer to control the spacing of the equipotential lines.

Ice Pat It is another object of the present invention to provide improved conductive shields around Ia high voltage transformer for spacing equipotential lines.

The invention will be best understood by reference to the accompanying drawing of which:

FIGURE 1 is a section view of the primary and secondary coils, and the associated electric field shaping structures, of a high voltage transformer,

FIGURE 2 is an enlarged View of a portion of the secondary winding in FIGURE l which is encircled by line 2 thereof,

FIGURE 3 is a general view of two of the coil structures of FIGURE 1 on an iron core, and

FIGURE 4 is a diagrammatic section view of the secondary coil with electric field equipotential lines shown thereon.

Referring now to FIGURE 1, there is shown the coil structure of a high voltage .transformer in which there is a tubu-lar coil form 11, made of -an epoxy insulative material. A conventional primary winding 12, having input leads 13, is disposed around and is supported by the coil form 11. As an example, the various dimensions and winding specification will be provided for a 4.6 kilovolt ampere rated transformer having a 208 volt primary winding and a 53.5 kilovolt secondary Winding. The primary winding 12 in this embodiment of the invention is twelve and one-half inches long wi-th fifty-two turns. A first conductive foil shield 14 is disposed around the primary winding 12 and extends for a short distance past each end thereof. A major insulation layer 16, disposed around the outside of the foil shield 14, is comprised of twentyfive sheets of .020 inch thick mica.

The major insulation 16 electrically isolates the primary winding 12 from a secondary winding 17 which has a configuration, in axial section, that approximates a trapezoid. Referring to FIGURE 2 in conjunction with FIGURE 1, the secondary coil 17 is wound in solenoidal type layers 15 with fewer turns per layer with increase in radius. In a conventional coil having a rectangular crosssection, the equipotentia-l lines tend to be concentrated near the outer radius of the coil. The trapezoidal coil shape of the present invention tends to equalize the spacing between equipotential lines since there are fewer turns per layer at the outer radius than at the inner radius. Thus the voltage per layer and the equipotential line distributionv is influenced by such configuration. An annular groove 18, with a triangular cross-section, is provided in the center of the lower portion 19 of the secondary winding 17 4to extend the width of such lower portion with respect to the upper portion. The space 18 is filled with a potting material Ztl such as an epoxy resin. Such shaping of the sides of the secondary winding 17 further aids in equally distributing the equipotential lines. The secondary winding 17 is comprised of many layers 15 of insulated wire 25, each layer being insulated from adjacent layers by sheets 21 of mica, each such sheet being longer than the adjacent winding layers. Thus the ends of each `sheet 21 extend outwardly beyond the sides of the secondary winding for a short distance. Such insulation pre vents breakdown between winding layers 15 while the projecting ends of the sheets 21 prevent breakdown around -tlie edges thereof.

-Not only must equalization of the spacing of equipotential lines along the edges of the secondary winding 17 be considered, -but also the equipotential line spacing at each end of the major insulation 16 must be equalized. The latter equipotential lines result from differences in potential between the primary and secondary windings 12 and 17. To aid in equalizing the spacing of such lines, a thin conductive shield 22 is disposed between the outer surface of the majo-r insulation 16 `and the inner surface of the secondary Winding 17, the start -or inner end 30 of the secondary winding 17 being electrically connected to the shield 22. The edges 23 of the shield 22 are flared outwar-dly from the major insulation 16. Thus, the shield 22 is in the form of a cylinder with flared ends 23. Edges 23 aid in shaping the equipotential line dis-tribution at the sides of .the secondary winding and at the ends of the space between the primary and secondary windings 12 and 17.

The outer annular surface -of the secondary winding 17 is also provided with a thin outer conductive foil shield 24 which is caused to encircle the outer-most surface of the perimeter of the secondary winding. A pair of corona shields 26 are disposed along each edge of the outer shield 24, each such corona shield having a semi-circular crosssectional configuration. Each shield 2,6 forms a ring around the perimeter of the secondary winding 17 with :the rounded side facing outwardly. The outer shield 24 is electrically connected to the finish end 35 of the secondary winding 17 and to the corona shields 26. All the shields 14, 22, 24 and 26 are made without forming an electrically complete annular conductor, the ends of the shields being mutually insulated so that a shorted turn is avoided. If an electrically complete turn were to be for-med, large quantities of short circuit current would flow in the shields.

` While the various shielding and winding shaping means discussed above for distributing the equipotential lines are .very useful, a major factor in obtaining the preferred eld distribution is the providing of potting material around the transformer in which an epoxy resin is divided into heterogeneous regions having differing dielectric constant. Barium titanate is added to various portions of the epoxy potting material to adjust the dielectric constant to lthe required value.

To distribute the equipotential lines at the secondary winding 17, a pair of epoxy rings 28 having high dielectric constant are each disposed coaxially around a separate one of the corona rings 26 along the outer surface of the secondary winding 17 and inwardly from the outer surface along approximately one-third of the side sur-face of .the winding. The rings 28 and the remainder of the secondary winding 17 is covered with a relatively low dielectric constant potting means 27. Typically, the dielectric constant of the rings 28 is twelve while that of the potting means 27 is four. The rings 28 and potting means 27 are generally cast under vacuum.

For equalizing the distribution of equipotential lines at the ends of t-he space between the primary and secondary windings 12 and 17, additional regions with controlled dielectric constant'are provided. In particular, rings 29 with a dielectric constant of twelve are disposed between the flared ends 23 of shield 22 and the major insulation 16. An additional pair of rings 31 with a dielectric constant of eight are disposed outwardly along the major insulation 16 from the rings 29 at opposite ends of the winding 17. The ring pairs 29 and 31 are enclosed by the previously described potting material 27. The shape of the potting material 27 conforms generally to' that of the secondary winding A17 and thus has a Itrapezoidal outline in section. The pot-ting means 27 also impregnates the windings 17, providing physical stability, a moisture barrier and electrical insulation.

Secondary winding leads 32 and 33 are provided, lead 32 being brought from the center of the outer shield 24 out through the potting means 27 while lead 33 is connected to one of the flared ends 23 of the field 22. Both leads 32 and 33 are insulated as indicated in FIGURE l, but in some instances it may be preferable to space the lead connections apart as shown in FIGURE 3 to reduce the danger of breakdown between the leads.

Two of the transformer assemblies of FIGURE 1 may be mounted on a rectangular iron core 41 as shown in FIGURE 3. The core'41 is of conventional laminated or wound construction. The transformer assemblies may be connected either in series or parallel.

In the operation of the transformer, an alternating potential is induced in the secondary winding 17 from the primary winding 12 by excitation in the conventional manner and intense voltage gradients are established both across the secondary winding and across the major insulation 16. In FIGURE 4, equipotential lines 36 are indicated on an outline drawing of the transformer, the relative voltages being indicated on a scale of 0 to 100 with the flared shield 22 being taken as the zero reference plane. The equipotential lines 36 are approximately evenly spaced at the interface between the potting material 27 and air which is the surface where breakdown is most likely to occur and thus the surface Where it is of greatest importance that equalized equipotential line spacing be provided.

The regions of differing dielectric constant alter the electric field distribution and thereby change the configuration of theequipotential lines. The regions of higher dielectric constant have higher capacitance and therefore the voltage drop across such regions is lower. Thus, such high dielectric constant regions tend to spread the equipotential lines. In the present invention, the dielectric constants of the various regions of the potting have been adjusted to obtain lthe preferred equipotential line distribution. Likewise, as previously discussed, the flaring of the shield 22 aids in distributing such lines, as does the trapezoidal shape of the secondary winding 17.

Obviously, a single transformer could be used instead of the two shown in FIGURE 3. For three phase operations, three transformer assemblies are used on one three legged core or three single core assemblies may be utilized.

While the invention has been disclosed with respect to a specific embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a high voltage transformer of the class having solid insulation, the combination comprising a core, an annular primary winding disposed 'on said core, a multilayered annular secondary winding having a progressively smaller axial dimension, a discontinuous annular conductive shield disposed between said primary and secondary windings in coaxial relation thereto, said shield having outwardly flaring ends projecting from between said primary and secondary windings, and an insulative potting material disposed around said primary and secondary windings and having regions with differing dielectric constants related to the dielectric stress of the adjacent areas of said secondary winding, the portions of said insulative potting material which are situated within said flared ends of said shield having a high dielectric constant relative to adjacent portions of said potting material.

2. In a high voltage transformer, the combination comprising a primary winding, a multi-layered annular secondary winding disposed around said primary winding, said secondary winding having a decreasing number of turns per layer with increase in distance from said primary winding, an annular discontinuous conductive shield disposed coaxially between said primary and said -secondary windings, said shield being longer in the axial direction than said secondary winding and having outwardly flared ends, insulative material disposed around said secondary winding and having a heterogeneous dielectric constant, said insulative material having regions of relatively high dielectric constant adjacent the layers of said secondary winding most distant from said primary winding and having a region of relatively high dielectric constant adjacent said flared ends of said shield and on the side of said shield facing said primary winding.

3. In a high voltage transformer having a magnetically permeable core material, the combination comprising a j Vprimary winding disposed around said core, a multilayered annular secondary winding disposed around said ina,

primary winding, the .turns per layer of said secondary winding decreasing with increasing distance of said layer from said primary winding, a conductive shield disposed between said primary and secondary windings and being connected tothe layer of ysaid secondary winding facing said primary winding, said shield having outwardly flared ends projecting from each end of said secondary winding, a first pair of annular insulators having a high dielectric constant and adjacent the outer layers of said secondary winding, a second pair of annular insulators having relatively high dielectric constant and 'disposed between said ared edges of said shield and said primary winding, and a third annular insulator encircling said rst and said second pair of insulators and said secondary coil and having a relatively low dielectric constant. j

4. Apparatus according lto claim 3 furt-her characterized in that the ratio of the dielectric constant of said first and said second pair of annular insulators to said third annular insulator is of the order of three to one.

5. Apparatus according to claim Sand comprising the further combination of a thick sheet ofinsulation disposed between said shield and said primary winding.

6. Apparatus according to claim 3fwherein the difference in axial length of adjacent layers, of turns of said secondary winding is greater at the inner portion of said winding than at the outer portion thereof, whereby the end surfaces of said secondary winding at the outermost layers form a larger angle with respect to the axis of the winding than do the end surfaces of the innermost layers.

7. In a high voltage transformer the combination comprising a magnetically permeable core, a primary winding disposedy around said core,`a thick sheet of insulation disposed around said primary winding, a multielayered secondary winding having a substantially trapezoidal crosssectional shape, said secondary winding being disposed aroundsaid sheet of insulation and having fewer turns per layer with increasing distance from said primary winda discontinuous annular conductive shield disposed between said secondary winding and said sheet of insulation and being electrically connected to the innermost layer of said secondary winding, said shield having outwardly flared ends projecting from said secondary winding, a first pair of annular insulators each having a relatively high dielectric constant encircling the outside surface of said secondary winding and each disposed along an adjacent -outer portion of an end of said secondary winding, a second pair of annular insulators each having a relatively high dielectric constant and disposed between said flared ends of said shield and said sheet of insulation, a third pair of annular insulators each having an intermediate value of dielectric constant and disposed adjacent said second pair of annular rings and ladjacent said sheet of insulation, a fourth annular insulator having a relatively low dielectric const-ant and encircling said first and second and third pair of annular insulators, said fourth insulator having a configuration generally following said shape of said secondary winding.

8. Apparatus as described in claim 7 wherein said first, second and third pairs of insulators and said fourth insulator -form a physically continuous thick layer of insulation around said secondary winding.

9. Apparatus according to claim 7 wherein said rst and second pairs of insulators have la dielectric constant of about twelve, said third pair of insulators has a dielectric con-stant of about eight and said fourth insulator has adielectric constant of about four.

References Cited by the Examiner UNITED STATES PATENTS LEWIS H. MYERS, Primary Examiner, ROBERT K. SCHAEFER, Examiner.

T. J. KOZMA, Assistant Examiner. 

1. IN A HIGH VOLTAGE TRANSFORMER OF THE CLASS HAVING SOLID INSULATION, THE COMBINATION COMPRISING A CORE, AN ANNULAR PRIMARY WINDING DISPOSED ON SAID CORE, A MULTILAYERED ANNULAR SECONDARY WINDING HAVING A PROGRESSIVELY SMALLER AXIAL DIMENSION, A DISCONTINUOUS ANNULAR CONDUCTIVE SHIELD DISPOSED BETWEEN SAID PRIMARY AND SECONDARY WINDINGS IN COAXIAL RELATION THEREOT, SAID SHIELD HAVING OUTWARDLY FLARING ENDS PROJECTING FROM BETWEEN SAID PRIMARY AND SECONDARY WINDINGS, AND AN INSULATIVE POTTING MATERIAL DISPOSED AROUND SAID PRIMARY AND SECONDARY WINDINGS AND HAVING REGIONS WITH DIFFERNIG DIELECTRIC CONSTANTS RELATED TO THE DIELECTRIC STRESS OF THE ADJACENT AREAS OF SAID SECONDARY WINDING, THE PORTIONS OF SAID INSULATIVE POTTING MATERIAL WHICH ARE SITUATED WITHIN SAID FLARED ENDS OF SAID SHIELD HAVING A HIGH DIELECTRIC CONSTANT RELATIVE TO ADJACENT PORTIONS OF SAID POTTING MATERIAL. 